fix: cherry-pick PR creation fails when PR author cannot be assigned #1050

Summary

When cherry-picking a PR to target branches, the gh pr create command includes --assignee {pr_author}. If the PR author is not a collaborator with appropriate permissions on the repository, GitHub rejects the assignee and the entire PR creation fails.

Problem / Motivation

This happened on RedHatQE/mtv-api-tests#371 where user rgolangh (who submitted via a fork) could not be assigned on RedHatQE/mtv-api-tests.

Error from logs:

Cherry pick PR creation failed: --- could not assign user: 'rgolangh' not found

Root Cause

In webhook_server/libs/handlers/runner_handler.py line 855:

assignee_flag = f" --assignee {shlex.quote(pr_author)}" if assign_to_pr_owner else ""

The --assignee flag is unconditionally set to the PR author without verifying they can be assigned.

Requirements

1. When assign_to_pr_owner is enabled, first attempt to use the PR author as assignee
2. If gh pr create fails due to assignee not found, retry without the --assignee flag (or with a fallback assignee from the OWNERS file / repository approvers)
3. The cherry-pick PR creation should not fail just because the author can't be assigned
4. Add test coverage for the fallback scenario

Deliverables

• Update cherry-pick PR creation to handle assignee failures gracefully
• Fall back to a maintainer/approver from the OWNERS file if the PR author cannot be assigned
• Add/update tests for the new behavior

Notes

The fix should be in webhook_server/libs/handlers/runner_handler.py in the cherry-pick PR creation logic. The fallback should be silent (log a warning, not post a PR comment about the fallback).